This invention relates to thyristors, and it relates, more particularly, to thyristors in a semiconductor body having a central zone of the first conductivity type and a predetermined doping level, at least one cathode-side base zone and at least one anode-side emitter zone of the second conductivity type, as well as at least one cathode electrode and one anode electrode.
Thyristors of this general type are widely known and used. Essential parameters of such thyristors are the maximum off-state voltage that can be withstood in the forward direction and in the reverse direction. This off-state voltage value is primarily a function of the doping and of the thickness of the central zone of the thyristor. However, the thickness of the central zone cannot be fully utilized. If, starting from the pn junction biased in blocking or reverse direction, the space charge region approaches the opposite pn junction, this produces an increase of the off-state current that is due to the fact that charge carrier pairs generated in the vicinity of the pn junction biased in the forward direction or reverse direction, respectively, act as control current for the pnp transistor structural portion of the thyristor.
In FIG. 1, a cross-sectional view of the semiconductor body of a typical conventional thyristor is illustrated schematically. The thyristor includes a central zone 1, an anode-side emitter zone 2, a cathode-side base zone 3, and a cathode-side emitter zone 4. If the off-state voltage in the breakover direction is present at the thyristor, the space charge region (shown hatched) expands from the pn junction between zones 1 and 3 in the direction of the pn junction between zones 1 and 2. In this case, the pn junction between zones 1 and 2 acts as an emitter for the pnp transistor type structure consisting of the zones 3, 1, 2. Negative charge carriers move out of zone 1 to zone 2, positive charge carriers out of zone 2 into zone 1. As the space charge region approaches the pn junction between zones 1 and 2, the greater will be the increase in off-state current, which is also intensified by transistor effect. The limit of blocking ability is reached when this current increases into steep rise.
When an off-state voltage is applied in the other direction (reverse direction), the pn junction between zones 1 and 2 is biased in the blocking direction. Then the pn junction between zones 1 and 3 is biased in forward direction and the space charge region must maintain a certain distance from this pn junction if punch through breakdown is to be avoided.
For this reason, the central zone must be made thicker than would be necessary for obtaining the desired breakdown voltage. Enlarging the width of the central zone, however, involves an increase of the on-state power losses and an increase of the carrier storage charge. The latter increases the recovery time and the reverse current integral.